Process for preparing spinel type lithium manganese composite oxide and cathode active material for rechargeable battery

ABSTRACT

A process for preparing a spinel lithium manganese complex oxide having the general formula Li x Mn 2−y Al y O 4  (wherein 0.9≦x≦1.1, and 0.002≦y≦0.5) the process comprising the steps of reacting a manganese complex hydroxide represented by the general formula (IIa) Mn 2+   (1−a) Al 3+   a (OH) [2+a−nz] (A n− ) 2 ·mH 2 O (wherein A n−  is an anion having a valence n, 0.001≦a≦0.25, 0.03&lt;z&lt;0.3 and 0&lt;m) with a water-soluble lithium compound in a molar ratio of Li/(Mn+Al) of 0.45˜0.55 in an aqueous medium to obtain a slurry, spray- or freeze-drying the obtained slurry, and heating the resultant dry material at a temperature of 600˜900° C.

TECHNICAL FIELD

The present invention relates to a process for preparing a spinel typeof lithium manganese complex oxide having an improved charging anddischarging cycle characteristics at an elevated temperature (50° C. ormore) and to a positive electrode active material for a lithium ionsecondary battery which comprises the spinel type of lithium manganesecomplex oxide prepared by this process.

BACKGROUND ART

As electronic appliances have been rendered small and portable in recentyears, there has been increased a demand for lithium secondary batterieshaving a light weight and a high energy density. Among them, a spineltype of LiMn₂O₄ which may be utilized as a positive electrode activematerial for 4 V grade of lithium ion secondary batteries has been drawnattention for the use as the battery material for general use productsuch as automobile and the like since the raw material for manganese isabundant in resource and inexpensive.

Although spinel type of LiMn₂O₄ is a material having relatively highbattery capacity, it has a problem in cycle characteristics duringcharge and discharge, especially cycle characteristics under an elevatedtemperature (50° C. or more) or in the elution of Mn into anelectrolyte.

In order to solve such a problem, there has been proposed substitutionof a part of Mn present in the spinel LiMn₂O₄ structure for other metal.MnO₆ octahedron wherein oxygen atoms are arranged in 6 directions isstructually unstable because Mn located at the center of the structurehas a large ionic radius so that clearance between oxygen atoms whichare located at the outside of the octahedron becomes much. On thecontrary, by substituting a part of Mn ion located at the center of theoctahedron for an ion which is smaller than Mn, the structure isstabilized for the reason that the clearance between oxygen atomsbecomes little so that a good packing is realized. Furthermore, it isexpected that the reactivity of the other metal substituted producttoward electrolyte is lowered so that the elution of Mn into theelectrolyte becomes remarkably little.

As a process for preparing Mn complex oxides wherein a part of Mn issubstituted for another metal, there is disclosed Al-substitutionprocess (Japanese Laid-Open Patent Application Nos. Hei 4-289662, Hei2-220358 etc.), B-substitution process (Japanese Laid-Open PatentApplication No. Hei 8-195200), Fe- and Cr-substitution process (JapaneseLaid-Open Patent Application No. Hei 9-245836) or the like. However,since each of these prior art processes is the so-called dry processwherein a water-insoluble Mn oxide or hydroxide and other metalcomponent are mixed in a powdered form and subsequently heated, therehas not yet been obtained complex oxide wherein metal to be substitutedis uniformly doped with Mn. Therefore, there has not yet been obtainedcomplex oxide which had eliminated sufficiently such fault of LiMn₂O₄ asin the charging and discharging cycle characteristics, especially cyclecharacteristics at an elevated temperature (50° C. or more) or in theelution of Mn into an electrolyte.

Also, although Mn series of secondary batteries have been drawnattention as the electric source material for use in an electricautomobile or notebook type of personal computer, they are often usedunder an environment of relatively higher temperature than the ordinarytemperature, and therefore, the improvement in the charging anddischarging cycle characteristics at an elevated temperature has beendesired.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a process for preparinga spinel type of lithium manganese complex oxide which has the improvedcharging and discharging cycle characteristics under an environment ofan elevated temperature (50° C. or more) by a means of substituting apart of Mn for the other metal and doping them uniformly as the solidsolution, and further to provie a positive electrode active material fora lithium ion secondary battery which comprises the spinel type oflithium manganese complex oxide prepared by this process.

As a result of having studied ardently to achieve the above objects, thepresent inventors have found that the above objects may be achieved byspinel type of lithium manganese complex oxides which may be obtained bythe process of the present invention and which may be represented by thegeneral formula (I)

Li_(x)Mn_((2−y))M_(y1)B_(y2)O₄  (I)

(wherein M is at least one metal selected from the group consisting ofAl, Cr, Fe, Ni, Co, Ga and Mg, x is 0.9≦x≦1.1, y represents y=y₁+y₂,0.002≦y≦0.5, 0≦y₁<0.5, 0.002≦y₂≦0.1) and which may also be representedby the general formula (Ia) where y₂ in the above general formula (I) is0

Li_(x)Mn_((2−y))M_(y)O₄  (Ia)

(wherein each of M and x has the same significance as the above and yrepresents 0.002≦y≦0.5), and further found that by using as the positiveelectrode active material for a lithium ion secondary battery thesespinel lithium manganese complex oxides wherein a part of Mn issubstituted for at least one metal selected from the group consisting ofAl, Cr, Fe, Ni, Co, Ga and Mg to result in a uniform doped product, thecharging and discharging cycle characteristics under an environment of arelatively higher temperature (50° C. or more) than the ordinarytemperature may be improved. The present invention is based on thesefindings.

That is, the present invention is a process for preparing a spinel typeof lithium manganese complex oxide which may be represented by thegeneral formula (I)

Li_(x)Mn_((2−y))M_(y1)B_(y2)O₄  (I)

(wherein M is at least one metal selected from the group consisting ofAl, Cr, Fe, Ni, Co, Ga and Mg, x is 0.9≦x≦1.1, y represents y=y₁₊y₂,0.002≦y≦0.5, 0≦y₁<0.5, 0.002≦y₂≦0.1) and which is characterized by thesteps of:

suspending in an aqueous medium a manganese complex hydroxide which maybe represented by the general formula (II)

Mn²⁺ _((1−a))M^(p+) _(a)(OH)_([2−(2−p)a−nz])(A^(n−))_(z) ·mH₂O  (II)

(wherein M^(p+) is at least one metal cation having p-valence (p=2˜3)selected from the group consisting of Al³⁺, Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Ga³⁺and Mg²⁺, A^(n−) is an anion having n-valence (n=1˜2) such as NO₃ ⁻,Cl⁻, Br⁻, CO₃ ²⁻, SO₄ ²⁻, CH₃COO⁻ or the like, a, z and m represent0≦a≦0.25, 0.03<z<0.3 and 0<m, respectively)

adding an aqueous solution containing boric acid ion to the suspension,

adding further a water-soluble lithium compound in such an amount as togive the molar ratio of Li/(Mn+B)=0.45˜0.55 [provided that the generalformula (II) is limited to one where a=0] which indicates a molar ratioof lithium to the total metal content in the aqueous medium or the molarratio of Li/(Mn+M+B)=0.45˜0.55 which indicates a molar ratio of lithiumto the total metal content in the aqueous medium to obtain a slurry,

spray- or freeze-drying the obtained slurry, and

heating the resultant residue at a temperature of 600 ˜900° C., and afurther process for preparing a spinel type of lithium manganese complexoxide which may be represented by the general formula (Ia) where y₂ inthe above general formula (I) is 0

Li_(x)Mn_((2−y))M_(y)O₄  (Ia)

(wherein each of M and x has the same significance, and y represents0.002≦y≦0.5) which is characterized by the steps of:

mixing a manganese complex hydroxide which may be represented by thegeneral formula (IIa)

Mn²⁺ _((1−a))M^(p+) _(a)(OH)_([2−(2−p)a−nz])(A^(n−))_(z)·mH₂O  (IIa)

(wherein M^(p+) is at least one metal cation having p-valence (p=2˜3)selected from the group consisting of Al³⁺, Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Ga³⁺and Mg²⁺, A^(n−) is an anion having n-valence (n=1˜2) such as NO₃ ⁻,Cl⁻, Br⁻, CO₃ ⁻, SO₄ ²⁻, CH₃COO⁻ or the like, a, z and m represent0≦a≦0.25, 0.03<z<0.3 and 0<m, respectively) with a water-soluble lithiumcompound in a molar ratio of Li/(Mn+M)=0.45˜0.55 in an aqueous medium toobtain a slurry,

spray- or freeze-drying the obtained slurry, and

heating the resultant residue at a temperature of 600 ˜900° C.

The process of the present invention has two features as stated below:

The first feature is to synthesize the manganese complex hydroxide whichmay be represented by the general formula (II) or (IIa) wherein themetal represented by M in the above, i.e. at least one kind of metalselected from the group consisting of Al, Cr, Fe, Ni, Co, Ga and Mg isdoped or bonded with manganese hydroxide in connection with the generalformula (I) where y₁≠0 or the general formula (Ia). By synthesizing sucha manganese complex hydroxide, there can be conducted heating at leastone kind of metal cation represented by the above M together with Mn notyet heated in uniformly doped or bonded state. Also, it is to synthesizea manganese complex hydroxide which may be represented byMn²⁺(OH)_([2−nz])(A^(n−))_(z)·mH₂O among the compounds represented bythe general formula (II) in connection with the general formula (I)where y₁=1.

In connection with the general formula (I), the manganese complexhydroxide obtained by the above synthesis process is filtered, washedwith water and suspended in water, and subsequently to the suspension isadded an aqueous solution containing boric acid ion thereby there may beprepared a slurry containing B. By this preparation process, B may bemixed uniformly with the above manganese complex hydroxide being as fineas the primary particle. Furthermore, if a water-soluble lithiumcompound is added to the above B-containing slurry in an aqueous medium,dried and heated, then there may be easily obtained the uniformly dopedspinel type of lithium manganese complex oxide which may be representedby the general formula (I).

In connection with the general formula (Ia), the manganese complexhydroxide obtained by this synthesis process is filtered, washed withwater and suspended in water, and subsequently to the suspension isadded a water soluble lithium compound in an aqueous medium wherebythere may be easily obtained the uniformly doped spinel type of lithiummanganese complex oxide which may be represented by the general formula(Ia).

The complex oxide of the present invention is one which has improvedremarkably cycle characteristics, especially charging and dischargingcycle characteristics under an environment of an elevated temperature(specifically 50° C. or more) which is the default of LiMn₂O₄, and theproblems as stated above have first been solved by using it as apositive electrode active material for a lithium ion secondary battery.

The second feature is to dry the slurry prepared from the above statedmanganese complex hydroxide (II) or (IIa) and the water soluble lithiumcompound by a spray- or freeze-drying. By application of thespray-drying which comprises drying the slurry with a nozzle ordiskatomizer, drying can be instantaneously accomplished so that themanganese complex hydroxide (II) or (IIa), boric acid ion and awater-soluble lithium compound remain dispersed at the level of therespective primary particles during drying to result in the formation ofa uniform composition. Also, by a high heat energe generated duringdrying, a very good precursor of the compound (I) or (Ia) may beobtained. By heating it, accordingly, the desired spinel and uniformdoped product may be obtained without accompanying the formation ofimprurities such as Mn₂O₃ and the like, and there may be obtaineduniformly doped product having improved cycle characteristics,especially charging and discharging cycle characteristics under anelevated temperature like the first feature. Even by spray-drying, amanganese complex hydroxide (II) or (IIa) and a water-soluble lithiumcompound remain uniformly dispersed at the level of the respectiveprimary particles so that they may be dried in the state of highlyuniform composition and hence a very good precursor of the complex oxiderepesented by the general formula (I) or (Ia) may be obtained.

As a process for drying the slurry, spray- or freeze-drying process maybe taken. However, the spray drying process where drying can beinstantaneously accomplished to provide the spherical material in alarge amount is preferred from the viewpoint of the sphericalgranulation nature and the uniformity of the composition.

For purpose of increasing the bulk density of the dried materialaccording to the necessity, it may also be heated after formed bycompression molding. As the dried material for compression molding,spray dried product is particularly preferred from an industrial aspect.

An atmosphere under which procedures of up to drying are performed isnot limited particularly. If desired, however, they may be performedunder a non-oxidative atmosphere, for example under a nitrogenatmosphere. When performed under a non-oxidative atmosphere, theformation of Mn³⁺ may be suppressed thereby obtaining the desired oxidedoped product may be easily obtained. Also, they may be conducted in thepresence of reducing agents, for example inorganic reduing agents suchas lithium borohydride, potassium borohydride, sodium borohydride andthe like; and organic reduing agents such as as ascorbic acid and thelike whereby there may be easily obtained the desired oxide dopedproduct with suppressed formation of Mn³⁺, as conducted under anon-oxidative atmosphere. Although the amount added of the reducingagent is not restricted particularly, usually it may be 1-10% by weightper the reaction volume.

As the water-soluble lithium compound, there may be preferably employedone containing an anion which is evaporated off during the heating. Forexample, lithium hydroxide, lithium nitrate, lithium oxalate or thesehydrates are preferred as the water-soluble lithium compound becausethey have an anion capable of evaporating off during the heating. Inthis invention, one or more of these compounds may be optionallyselected for use.

As boron, boric acid may be taken. Boric acid may be easily dissolved inwater.

The manganese complex hydroxide represented by the general formula (II)or (IIa) which may be used for preparing the spinel type of complexoxide represented by the general formula (I) or (Ia) may be obtained bythe following preparation process.

That is, in the cases of the general formula (II) where a≠0 and thegeneral formula (IIa), in the cases of the general formula (II) wherea≠0 and the general formula (IIa), a mixed solution comprising awater-soluble Mn and a water-soluble salt of metal represented by Mwherein the atomic ratio of M/(Mn+M) becomes 0.001-0.25 in aqueousmedium is prepared, and then the mixed solution and an aqueous alkalinesolution are simultaneously added so as to give pH 8 or more therebyeffecting the reaction. The reaction product obtained is filtered andwashed with water. Otherwise, in the case of the general formula (II)where a=0, an aqueous solution of a water-suluble salt of Mn is preparedand then an alkali is added dropwisely to the aqueous solution while pHof the system is kept to 8 or more thereby effecting the reaction. Thereaction product obtained is filtered and washed with water. Theresultant product may be used for the subsequent reaction as it is or inthe form of being suspended in water. It is particularly preferable toconduct the above-stated reaction under a non-oxidative atomosphere, forexample with bubbling of nitrogen, under a vigorous stirring withmaintaining the reaction temperature to 25° C. and adjusting theresident time for 30 minutes.

Examples of the alkali to be used in the present invention includealkali metal hydroxides such as sodium hydroxide, potassium hydroxide,lithium hydroxide and the like. Of them, sodium hydroxide is preferable.

A^(n−) in the general formula (I) or (Ia) indicates an anion havingn-valence. For example, there may be taken anions such as NO₃ ⁻, Cl⁻,Br⁻, CH₃COO⁻, CO₃ ²⁻, SO₄ ²⁻ and the like.

A wide variety of metal compounds which may be represented by M are notrestricted particularly so long as they are water-soluble. For example,metal carbonate, metal nitrate, one derived from organo metal complexesand the like may be taken. Preferable ones are shown below.

As water-soluble salts of manganese there may be taken manganesesulfate, manganese nitrate, manganese chloride and the like, aswater-soluble salts of Al aluminum chloride and the like, aswater-soluble salts of Cr chromium nitrate 9 hydrate and the like, aswater-soluble salts of Fe iron chloride, iron nitrate 9 hydrate and thelike, as water-soluble salt of Ni nickel nitrate, as a water-solublesalt of Co cobalt nitrate 6 hydrate, as a water-soluble salt of Gagailium nitrate and as a water-soluble salt of Mg magnesium chloride andthe like.

The manganese complex hydroxide represented by the general formula (II)or (IIa) which may be used as the starting material in the presentinvention may also be prepared by the so-called seed circilationreaction method as stated below.

That is, an aqueous solution comprising a water-soluble salt of Mn and awater-soluble of M (at least one of Al, Cr, Fe, Ni, Co, Ga and Mg) or awater-soluble salt of Mn and an aqueous alkaline solution are suppliedin the predetermined amount through the respective quantitative pumps toa reaction vessel equipped with a overflow to effect a reaction under astirring at a pH of 8 or more, the overflowed slurry of the reactionproduct is transported to a thickner and the concentrated slurry in thethickner is contineously returned as seed to the reaction vessel.According to this preparation process, the doped manganese complexhydroxide having a bulk density of about 2 or more may be easilyprepared.

The heating may be conducted under an atmosphere of an excess of oxygenin air. However, preferably it may be conducted in air which is moreeconomical.

The temperature at which the heating is conducted may be 600˜900° C.,preferably 700˜800° C. and more preferably 750° C. At a temperaturebelow 600° C., there can not be obtained a product having a sufficientbattery capacity while at a temperature exceeding 900° C. a product issusceptible to decomposition. Any particular control is not required fora heating atmosphere, and it is enough to conduct under an atmosphere ofan air.

The heating time is not rectricted particularly. However, it may be 1hour or more, preferably 5˜20 hours, and more preferably about 10 hours.If desired, the dried materal may be heated after molded by thecompression molding and the like in order to increase the bulk densityof the heated material.

The thus obtained spinel type of lithium manganese complex oxide of thepresent invention which may be represented by. the general formula (I)or (Ia):

Li_(x)Mn_((2−y))M_(y1)B_(y2)O₄  (I)

Li_(x)Mn_((2−y))M_(y)O₄  (Ia)

may be effectively utilized as a positive electrode active material fora lithium ion secondary battery.

The battery test for the positive electrode active material of thepresent invention has been conducted by preparing a test cell andsubjecting it to the test according to the method in ExperimentalExample as described later. The improvement in charging and dischargingcharaceristics under an environment of an elevated temperature (50° C.or more) has been confirmed by placing the test cell in aconstant-temperature oven heated to 60° C. and then subjecting it tocharge and discharge.

As a negative electrode for the lithium ion secondary battery whereinthe spinel type of lithium manganese complex oxide of the presentinvention has been used as the positive electrode active material, theremay be used lithium metal, lithium alloy and a compound which is capableof occluding and releasing lithium reversibly. Examples of lithium alloyinclude lithium/tin alloy, lithium/aluminum alloy, lithium/lead alloyand the like. Examples of the compound which is capable of occluding andreleasing lithium reversibly include carbon materials such as blackcarbon, graphite and the like.

An electrolyte is not restricted particularly but there may be used atleast one of organic solvents, for example carbonates such as propylenecarbonate, diethyl carbonate and the like; sulfolanes such as sulfolane,dimethylsulfoxide and the like; lactones such as γ-butyrolactone and thelike; and ethers such as dimethoxyethane and the like.

Electrolytes are not restricted particularly but there may also be usedat least one of lithium salts such as lithium perchlorate, lithiumtetrafluoroborate, lithium hexafluoro phosphate, lithiumtrifluoromethanesulfonate, said lithium salts being dissolved in thesolvents as stated above; and an inorganic or organic solid electrolytehaving lithium ion conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is powder X-ray diffraction patterns of the lithium manganesecomplex oxides obtained in Example 1 and Comparative Example 1.

FIG. 2 is powder X-ray diffraction patterns of the lithium manganesecomplex oxides obtained in Examples 5˜8 and Comparative Example 3.

FIG. 3 is powder X-ray diffraction patterns of the lithium manganesecomplex oxides obtained in Examples 9˜12 and Comparative Example 4.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention is more specifically illustrated by the followingexamples.

EXAMPLE 1

There was prepared 100 ml of a mixed aqueous solution of 2.0 mol/lmanganese nitrate and aluminum nitrate wherein the atomic ratio ofAl/(Mn+Al) was 0.05. This mixed aqueous solution was added to 100 ml of4.0 mol/l aqueous sodium hydroxide solution. The reaction was conductedunder a vigorous stirring while a nitrogen gas was bubbled therein andthe temperature was maintained to 25° C.

The obtained reaction solution was filtered out under an atmosphere of anitrogen gas and washed with water, and thereafter suspended in waterthereby obtaining a slurry of 1 mol/l compound represented by thecomposition formulaMn_(0.941)Al_(0.051)(OH)_(1.988)(NO₃)_(0.047)·0.10H₂O (this compositionformula was identified from a portion which has been taken from thereaction product and dried at 110° C. an overnight, and similarly refersto other Examples). To the slurry 3.0 mol/l aqueous lithium hydroxidesolution was added dropwisely in an amount such that the atomic ratio ofLi to the (Mn+Al) content in the slurry i.e. Li/(Mn+Al) became 0.51 toeffect the reaction. Thereafter, the reaction mixture was spray-dried. Aseries of procedures of from washing to drying was conducted in anenvironment where non-oxidative atmosphere (nitrogen atmosphere) waskept.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace (TF-630 type, aproduct of Yamada Electric Company), and the heating product waspulverized in an agate mortar to obtain powders represented by thecomposition formula of Li_(0.997)Mn_(1.907)Al_(0.093)O₄.

All of the X-ray peaks appearing in the X-ray diffraction pattern of theobtained powder were shown to be attributed to the spinel LiMn₂O₄. Thelattice constant was 8.16 angstroms.

EXAMPLE 2

Ascorbic acid was added in an amount of 2% (w/v) to a mixed aqueoussolution of 2.0 mol/l of manganese sulfate and aluminum sulfate whereinthe atomic ratio of Al/(Mn+Al) was 0.05. In the addition to the thusprepared mixed solution, 2.5 mol/l of aqueous sodium carbonate solutionwas prepared. The above-mentioned 2 kinds of solutions and 4.0 mol/laqueous sodium hydroxide solution were simultaneously added so as tomaintain pH 9.0, whereupon the continuous reaction was conducted at 25°C. for 30 minutes of the resident time. The obtained reaction solutionwas continuously fed to a thickener, concentrated and returned to thereaction vessel at 20 times velocity as compared with the additionvelocity of the raw material to carry out the so-called seed circulationreaction.

The obtained reaction solution was filtered, washed with water andsuspended in water, thereby obtaining a slurry of 1 mol/l compoundrepresented by the composition formulaMn_(0.920)Al_(0.080)(OH)_(2.000)(CO₃)_(0.040)·0.11H₂O. To thissuspension were added dropwisely 1.0 mol/l H₃BO₃ aqueous solution and3.0 mol/l aqueous lithium hydroxide solution in the respective amountssuch that the atomic ratio of B to the (Mn+Al) content in the suspensioni.e. B/(Mn+Al) became 0.02 and that the atomic ratio of Li/(Mn+Al+B)became 0.51 to effect the reaction. Thereafter, the reaction mixture wasspray dried. The obtained dry gel was put in an alumina boat and heatedat 750° C. for 10 hours in an atmosphere of air in a tube furnace(TF-630 type, a product of Yamada Electric Company), and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.010)Mn_(1.800)Al_(0.016)B_(0.04)O₄.

All of the X-ray peaks appearing in the X-ray diffraction pattern of theobtained powder were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.15 angstroms.

EXAMPLE 3

Similar procedure as in Example 2 was conducted except that no Al saltwas used. That is, ascorbic acid was added to 2.0 mol/l of aqueousmanganese sulfate solution in an amount of 2% (w/v) based on the volumeof the aqueous solution. In addition to this aqueous solution, 2.5 mol/lof aqueous sodium carbonate solution was prepared. The above-mentioned 2kinds of solutions and 4.0 mol/l aqueous sodium hydroxide solution weresimultaneously added so as to maintain pH 9.0, whereupon the continuousreaction was conducted at 25° C. for 30 minutes of the resident time.The obtained reaction solution was continuously fed to a thickener,concentrated and returned to the reaction vessel at 20 times velocity ascompared with the addition velocity of the raw material to carry out theso-called seed circulation reaction.

The obtained reaction solution was filtered, washed with water andsuspended in water, thereby obtaining a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(1.000)(OH)_(1.954)(CO₃)_(0.023)·0.09H₂O. To this suspension wereadded dropwisely 1.0 mol/l H₃BO₃ aqueous solution and 3.0 mol/l aqueouslithium hydroxide solution in the respective amount such that the atomicratio of B to the Mn content in the suspension i.e. B/Mn became 0.02 andthat the atomic ratio of Li/(Mn+B) became 0.51 to effect the reaction.Thereafter, the reaction mixture was spray dried. The obtained dry gelwas put in an alumina boat and heated at 750° C. for 10 hours in anatmosphere of air in a tube furnace (TF-630 type, a product of YamadaElectric Company), and the heating product was pulverized in an agatemortar to obtain powders represented by the composition formula thecomposition formula of Li_(0.013)Mn_(1.960)B_(0.04)O₄.

All of the X-ray peaks appearing in the X-ray diffraction pattern of theobtained powder were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.17 angstroms.

Comparative Example 1

Each of sodium hydroxide monohydrate, manganese dioxide and aluminumnitrate (special class of reagent prepared by Wako Pure ChemicalIndustries, Ltd. was each used) weighed so as to give the atomic ratioof Al/(Mn+Al)=0.05 and the atomic ratio of Li/(Mn+Al)=0.51 and was mixedtogether sufficiently in an agate mortor. Thereafter, dry gel was put inan alumina boat and heated at 900° C. for 10 hours in an atmosphere ofair in a tube furnace, and the heating product was pulverized in anagate mortar to obtain powders represented by the composition formula ofLi_(1.004)Mn_(1.903)Al_(0.097)O₄.

Some X-ray peaks attributing to the spinel LiMn₂O₄ were observed in theX-ray diffraction pattern of the obtained powder. The lattice constantwas 8.20 angstroms.

Comparative Example 2

Each of sodium hydroxide monohydrate, manganese dioxide and boron oxide(special class of reagent prepared by Wako Pure Chemical Industries,Ltd. was each used) weighed so as to give the atomic ratio ofB/(Mn+B)=0.04 and the atomic ratio of Li/(Mn+B)=0.51 and was mixedtogether sufficiently in an agate mortor. Thereafter, dry gel was put inan alumina boat and heated at 900° C. for 10 hours in an atmosphere ofair in a tube furnace, and the heating product was pulverized in anagate mortar to obtain powders represented by the composition formula ofLi_(1.013)Mn_(1.960)B_(0.040)O₄.

Some X-ray peaks attributing to Li₂B₄O₇ in addition to ones attributingto the spinel LiMn₂O₄ were observed in the X-ray diffraction pattern ofthe obtained powder. The lattice constant was 8.20 angstroms.

Experimental Example 1

The following battery test (charge and discharge test) was conductedusing each of complex oxides of Example 1 and Comparative Example 1 asstated above:

For use as the positive electrode material 25 mg ofLi_(x)Mn_((2−y))M_(y)O₄ obtained in the above Example and ComparativeExample was mixed with 16 mg of TAB -2 as a conductive agent and themixture was subjected to compression molding on stainless steel mesh,thereby obtaining pellets each having a diammeter of 18 mm. The obtainedpellets were dried at 200° C. for 2 hours or more to prepare a positiveelectrode material.

As a negative electrode material was used a rolled lithium metal sheetpressed-bonded to a stainless substrate. As a diaphragm, porous membranemade of polypropylene (Cell Guard 2502) and a glass filter-paper filterwere used. There was used an electrolyte in which 1 mol/l LiPF₆ wasdissolved in an ethylene carbonate/dimethyl carbonate mixture (1:2). Theprocedures of up to finish from setup of a test cell (semi-open typecell) were conducted in an argon replaced dry box. The charging anddischarging for this test cell were performed under a voltage controlbetween 3.5 V and 4.5 V at a constant current density of 0.4 mA/cm².

As a result of this battery test, the initial discharge capacity(mAh/g), discharge capacity at 50th cycle (mAh/g) and decrease rate ofdischarge capacity at 50th cycle (%) were as shown in table 1.

TABLE 1 Initial Discharge Discharge Capacity Decrease Rate of SampleCapacity at 50th Cycle Discharge Capacity No. (mAh/g) (mAh/g) at 50thCycle (%) Example 1 112.8 111.2 1.4 114.5 113.6 0.6 113.5 112.4 1.0 Com-90.6 63.8 29.8 parative 74.5 68.2 8.5 Example 1 83.7 72.5 13.4

Comparative Example 3

1.0 Mol/l aqueous manganese nitrate solution was prepared and then thisaqueous solution and 1.0 mol/l aqueous sodium hydroxide solution wereadded simultaneously so as to maintain pH 9.0, whereupon the continuousreaction was conducted. The reaction was conducted under a vigorousstirring with bubbling a nitrogen gas while the temperature and the theresident time were maintained to 25° C. and 30 minutes, respectively.The obtained reaction solution was filtered, washed with water, andsuspended in water thereby obtaining a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(1.000)(OH)_(1.949)(NO₃)_(0.051)·0.10H₂O.

To the suspension 3.0 mol/l aqueous lithium hydroxide solution was addeddropwisely in an amount such that the atomic ratio of Li/Mn became 0.51with respect to the Mn content in the suspension to effect the reaction.Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace (TF-630 type, aproduct of Yamada Electric Company), and the heating product waspulverized in an agate mortar to obtain powders represented by thecomposition formula of Li_(1.062)Mn_(1.954)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 2. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 4

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and aluminum nitrate wherein the atomic ratio of Al/(Mn+Al) was0.05. This mixed aqueous solution and 1.0 mol/l aqueous sodium hydroxidesolution were added simultaneously so as to give pH 11.0, whereupon thecontinuous reaction was conducted. The reaction was conducted under avigorous stirring and bubbling of a nitrogen gas while the temperatureand the resident time were maintained to 25° C. and 30 minutes,respectively. The obtained reaction solution was filtered, washed withwater, and thereafter suspended in water thereby obtaining a slurry of 1mol/l a compound represented by the composition formulaMn_(0.951)Al_(0.049)(OH)_(1.998)(NO₃)_(0.051)·0.12H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Al)became 0.51 with respect to the (Mn+Al) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace (TF-630 type, aproduct of Yamada Electric Company), and the heating product waspulverized in an agate mortar to obtain powders represented by thecomposition formula of Li_(1.997)Mn_(1.907)Al_(0.099)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 2. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.21 angstroms.

EXAMPLE 5

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and aluminum nitrate wherein the atomic ratio of Al/(Mn+Al) was0.05. This mixed aqueous solution and 1.0 mol/l aqueous sodium hydroxidesolution were added simultaneously so as to keep pH 11.0, whereupon thecontinuous reaction was conducted. The reaction was conducted under avigorous stirring and bubbling of a nitrogen gas while the reactiontemperature and the resident time were maintained to 25° C. and 30minutes, respectively. The obtained reaction solution was filtered,washed with water, and thereafter suspended in water thereby obtaining aslurry of 1 mol/l a compound represented by the composition formulaMn_(0.950)Al_(0.050)(OH)_(1.994)(NO₃)_(0.056)·0.10H₂O. To thissuspension were added dropwisely 1.0 mol/l H₃BO₃ aqueous solution and3.0 mol/l aqueous lithium hydroxide solution in the respective amountsuch that the atomic ratio of B/(Mn+Al) became 0.02 with respect to the(Mn+Al) content in the suspension and that the atomic ratio ofLi/(Mn+Al+B) became 0.51 to effect the reaction. Thereafter, thereaction mixture was spray-dried.

The obtained dry gel was pulverized in an agate mortar to obtain powdersrepresented by the composition formula ofLi_(1.013)Mn_(1.866)Al_(0.100)B_(0.040)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 2. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.21 angstroms.

EXAMPLE 6

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and chromium (III) nitrate wherein the atomic ratio ofCr/(Mn+Cr) was 0.03. The mixed aqueous solution was treated according tothe similar manner as in Example 4 to obtain a slurry of 1 mol/l acompound represented by the composition formulaMn_(0.970)Cr_(0.030)(OH)_(1.991)(NO₃)_(0.039)·0.12H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Cr)became 0.51 with respect to the (Mn+Cr) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.050)Mn_(1.924)Cr_(0.060)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 2. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 7

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and iron (III) nitrate wherein the atomic ratio of Fe/(Mn+Fe)was 0.03. The mixed aqueous solution was treated according to thesimilar manner as in Example 4 to obtain a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(0.970)Fe_(0.030)(OH)_(1.995)(NO₃)_(0.035)·0.10H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Fe)became 0.51 with respect to the (Mn+Fe) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.054)Mn_(1.876)Fe_(0.059)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 2. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 8

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and nickel nitrate wherein the atomic ratio of Ni/(Mn+Ni) was0.03. The mixed aqueous solution was treated according to the similarmanner as in Example 4 to obtain a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(0.971)Ni_(0.029)(OH)_(1.978)(NO₃)_(0.022)·0.12H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Ni)became 0.51 with respect to the (Mn+Ni) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.033)Mn_(1.904)Ni_(0.059)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 2. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 9

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and cobalt nitrate wherein the atomic ratio of Co/(Mn+Co) was0.03. The mixed aqueous solution was treated according to the similarmanner as in Example 4 to obtain a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(0.970)Co_(0.030)(OH)_(1.967)(NO₃)_(0.033)·0.11H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Co)became 0.51 with respect to the (Mn+Co) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace (TF-630 type, aproduct of Yamada Electric Company), and the heating product waspulverized in an agate mortar to obtain powders represented by thecomposition formula of Li_(1.022)Mn_(1.911)Co_(0.060)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 3. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 10

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and gallium nitrate wherein the atomic ratio of Ga/(Mn+Ga) was0.03. The mixed aqueous solution was treated according to the similarmanner as in Example 4 to obtain a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(0.969)Ga_(0.031)(OH)_(1.992)(NO₃)_(0.039)·0.10H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Ga)became 0.51 with respect to the (Mn+Ga) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.051)Mn_(1.896)Ga_(0.060)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 3. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 11

There was prepared a mixed aqueous solution of 1.0 mol/l manganesenitrate and magnesium nitrate wherein the atomic ratio of Mg/(Mn+Mg) was0.03. The mixed aqueous solution was treated according to the similarmanner as in Example 4 to obtain a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(0.971)Mg_(0.029)(OH)_(1.971)(NO₃)_(0.029)·0.12H₂O.

3.0 Mol/l aqueous lithium hydroxide solution was added dropwisely tothis suspension in an amount such that the atomic ratio of Li/(Mn+Mg)became 0.51 with respect to the (Mn+Mg) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray-dried.

The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.020)Mn_(1.928)Mg_(0.059)O₄.

The X-ray diffraction pattern of this powder was shown in FIG. 3. All ofthe X-ray peaks were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.23 angstroms.

EXAMPLE 12

There were prepared a mixed aqueous solution of 2.0 mol/l of manganesesulfate and aluminum sulfate wherein the atomic ratio of Al/(Mn+Al) was0.05 and 2.5 mol/l of aqueous sodium carbonate solution. Theabove-mentioned 2 kinds of solutions and 4.0 mol/l aqueous sodiumhydroxide solution were simultaneously added so as to maintain pH 9.0,whereupon the continuous reaction was conducted at 25° C. for 30 minutesof the resident time. The obtained reaction solution was continuouslyfed to a thickener, concentrated and returned to the reaction vessel at20 times velocity as compared with the addition velocity of the rawmaterial to carry out the so-called seed circulation reaction.

The obtained reaction solution was filtered, washed with water andsuspended in water, thereby obtaining a slurry of 1 mol/l a compoundrepresented by the composition formulaMn_(0.949)Al_(0.051)(OH)_(1.967)(CO₃)_(0.042)·0.11H₂O.

To this suspension was added dropwisely 3.0 mol/l aqueous lithiumhydroxide solution in an amount such that the atomic ratio of Li/(Mn+Al)became 0.51 with respect to the (Mn+Al) content in the suspension toeffect the reaction. Thereafter, the reaction mixture was spray dried.The obtained dry gel was put in an alumina boat and heated at 750° C.for 10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.013)Mn_(1.894)Al_(0.100)O₄.

All of the X-ray peaks appearing in the X-ray diffraction pattern of theobtained powder were attributed to the spinel LiMn₂O₄. The latticeconstant was 8.22 angstroms.

Comparative Example 4

Potassium hydroxide monohydrate, manganese dioxide and aluminum nitrateweighed and were mixed sufficiently in an agate mortor so as to give theatomic ratio of Al/(Mn+Al)=0.05 and the atomic ratio of Li/(Mn+Al)=0.51.Thereafter, dry gel was put in an alumina boat and heated at 900° C. for10 hours in an atmosphere of air in a tube furnace, and the heatingproduct was pulverized in an agate mortar to obtain powders representedby the composition formula of Li_(1.004)Mn_(1.902)Al_(0.097)O₄.

There were observed X-ray peaks attributing to Li₂MnO₃ and Mn₂O₃ as wellas those attributing to the spinel LiMn₂O₄ in the X-ray diffractionpattern of the obtained powder.

The lattice constant was 8.21 angstroms.

Experimental Example 2

The following battery test (charge and discharge test) was conductedusing the respective complex oxides of the above-stated Examples 5˜12and Comparative Example 4:

A test cell and an electrolyte for battery test was prepared by thesimilar manner as in Experimental Example 1.

The test cell was placed in a thermostatic oven of 60° C. and allowed tostand for 30 minutes in a open-circuit state. Thereafter, theconstant-current and -voltage charging and discharging were performedunder a voltage between 3.5 V and 4.5 V at a constant current density of0.4 mA/cm².

As a result of this battery test, the initial discharge capacity(mAh/g), discharge capacity at 50th cycle (mAh/g) and decrease rate ofdischarge capacity at 50th cycle (%) were as shown in table 2.

TABLE 2 Initial Discharge Decrease Discharge Capacity Discharge Rate ofBulk Sample Capacity at 50th Cycle at 50th Cycle Capacity Density No.(mAh/g) (mAh/g) (%) (g/ml) Comparative 131.8 95.6 27.19 1.51 Example 3Example 4 106.5 100.9 5.26 1.39 Example 5 103.6 96.5 6.85 1.37 Example 6120.7 111.7 7.45 1.64 Example 7 100.9 75.3 25.37 1.41 Example 8 103.9102.2 1.64 1.55 Example 9 123.7 99.2 19.81 1.62 Example 10 122.2 104.114.81 1.49 Example 11 117.5 101.9 13.28 1.43 Example 12 100.3 93.2 7.082.02 Comparative 98.3 68.5 30.32 1.42 Example 4

Industrial Applicability

According to the present invention there can be provided a novel processfor preparing a spinel type of lithium manganese complex oxide having animproved charging and discharging cycle characteristics under anenvironment of the elevated temperature (50° C. or higher) which may berepresented by the general formula (I)

Li_(x)Mn_((2−y))M_(y1)B_(y2)O₄  (I)

(wherein M represents at least one selected from the group consisting ofAl, Cr, Fe, Ni, Co, Ga and Mg, x represents 0.9≦x≦1.1, y representsy=y₁+y₂, 0.002≦y≦0.5, 0≦y₁<0.5, 0.002≦y₂≦0.1) and which may also berepresented by the general formula (Ia) where y₂ in the above generalformula (I) is 0

Li_(x)Mn_((2−y))M_(y)O₄  (Ia)

(wherein M and x have the same significance as the above and yrepresents 0.002≦y≦0.5) and a positive electrode active material for alithium ion secondary battery which comprises the complex oxide preparedby said process.

The complex oxide prepared by the present invention is industrially veryuseful since it has an improved cycle characteristics that has not beenachieved, especially an improved charging and discharging cyclecharacteristics at higher temperature (about 50° C. or more) than roomtemperature.

What is claimed is:
 1. A process for preparing a spinel lithiummanganese complex oxide represented by the general formula (Ia)Li_(x)Mn_(2−y)Al_(y)O₄  (Ia) (wherein 0.9<x<1.1, and 0.002<y<0.5), saidprocess comprising the steps of: reacting a manganese complex hydroxiderepresented by the general formula (IIa)  Mn²⁺ _((1−a))Al³⁺_(a)(OH)_([2+a−nz])(A^(n−))₂·mH₂O  (IIa) (wherein A^(n−) is an anionhaving a valence n, 0.001≦a≦0.25, 0.03<z<0.3 and 0<m) with awater-soluble lithium compound in a molar ratio of Li/(Mn+Al) of0.45˜0.55 in an aqueous medium to obtain a slurry, spray- orfreeze-drying the obtained slurry, and heating the resultant drymaterial at a temperature of 600˜900° C.
 2. A process for preparing aspinel lithium manganese complex oxide as claimed in claim 1 whereinsaid manganese complex hydroxide represented by the general formula(IIa) is one obtained by a process which comprises preparing a mixedaqueous solution comprising a water-soluble salt of Mn and awater-soluble salt of Al wherein an atomic ratio of Al/(Mn+Al) is in therange of 0.001˜0.25 in an aqueous medium, adding the mixed aqueoussolution simultaneously with an aqueous alkaline solution to a reactionvessel so as to give a pH of 8 or more to effect the reaction, filteringand washing the obtained reaction product with water.
 3. A process forpreparing a spinel lithium manganese complex oxide as claimed in claim 1wherein said manganese complex hydroxide represented by the generalformula (IIa) is one obtained by a process which comprises preparing amixed aqueous solution comprising a water-soluble salt of Mn and awater-soluble salt of Al wherein an atomic ratio of Al/(Mn+Al) is in therange of 0.001˜0.25 in an aqueous medium, and adding an aqueous alkalinesolution to the mixed aqueous solution under a stirring to effect areaction while a pH of the reaction is kept to 8 or more.
 4. A processfor preparing a spinel lithium manganese complex oxide as claimed inclaim 3 wherein a reaction to form the manganese complex hydroxiderepresented by the general formula (IIa) is conducted in a non-oxidativemanner.
 5. A process for preparing a spinel lithium manganese complexoxide as claimed in claim 3 wherein a reaction to form the manganesecomplex hydroxide represented by the general formula (IIa) is conductedin the presence of a reducing agent.
 6. A process for preparing a spinellithium manganese complex oxide as claimed in claim 2 wherein apreparation of the manganese complex hydroxide represented by thegeneral formula (IIa) has been accomplished by a process wherein a mixedaqueous solution comprising a water-soluble salt of Mn and awater-soluble salt of Al, and an alkaline aqueous solution are suppliedthrough the respective quantitative pumps to a reaction vessel equippedwith an overflow to effect a reaction under a stirring at a pH of 8 ormore, the overflowed slurry of the reaction product is transported to athickener and the slurry concentrated by the thickener is continuouslyreturned as seed to the reaction vessel.
 7. A positive electrode activematerial for a lithium ion secondary battery which comprises the spineltype of lithium manganese complex oxide prepared by the process asclaimed in claim
 1. 8. A positive electrode active material for alithium ion secondary battery as claimed in claim 7 wherein saidpositive electrode active material has an improved charging anddischarging characteristic property under an elevated temperature.